Class certification in legal cases involves using aggregate proof, which is evidence like statistics showing a common harm caused by the defendant. Courts are facing challenges in deciding on class certification due to conflicting expert opinions. The key issue is not about facts but about interpreting the law. By focusing on the court's role in interpreting the law, class certification decisions can be more consistent and clear. This approach has been applied in cases like the RICO class action against the tobacco industry and the employment discrimination class action against Wal-Mart.